The present invention relates to a streaking tube, and especially to a synchronous scan streaking device which is suitable for measuring repetitive pulses of diminished light of the same waveform in the same interval.
The streaking camera is a well known device for observing the light intensity distribution with time of light pulses which may change at high speed.
The streaking tube in the streaking camera is an electron tube consisting of a photoelectric layer, a phosphor layer, and a pair of deflection electrodes arranged between the photoelectric layer and the phosphor layer.
When the light is incident on the photoelectric layer of the streaking camera, the photoelectric layer emits photoelectrons in accordance with the incident light intensity, which may change with time, to form a photoelectron beam image.
When an electric field is applied across the deflection electrodes during transportation of photoelectrons toward the phosphor layer, the photoelectron beam is scanned in line on the phosphor layer and the incident light intensity change becomes the brightness change on the phosphor layer along the photoelectron beam scanning line (on the time coordinate).
The image on the phosphor layer is called the streaking image. This type of image is photographed or taken up by a TV camera to measure the brightness distribution along the scanning line. The light intensity change with time can thus be known.
The synchronous scan streaking device utilizing this type of streaking tube structure can be used to measure the repetitive pulses of diminished light.
This type of diminished repetitive light pulses is, for instance, a series of light pulses which have occurred in a fluorescent material excited by laser beam pulses.
When the measured light pulse intensity is very small, the resulting image intensity is very small and thus an accurate light intensity distribution cannot be obtained.
If the repetitively measured light pulses are of the same waveform repeated in the same period, the streaking image of the same intensity distribution along the scanning line (on the time coordinate) can duplicatedly be put on the same location of the phosphor layer when the sine-wave voltages in the same interval as the repetitive light pulses are applied to the streaking tube deflection electrodes in a predetermined phase relation with respect to the repetitive light pulses.
Brightness of the streaking image on the phosphor layer is enhanced by "n" times the single-scan brightness if the same image is generated "n" times during scanning. This results in a satisfactory streaking image with high S/N ratio even if the streaking image intensity is very small.
The synchronous scan streaking device is one in which the principle of operation is realized in a vacuum envelope.
The inventors found that the streaking image was distorted by a multipactoring discharge occurring in a streaking tube during measurement if the measurement was carried out by using a synchronous scan streaking device in the conventional device.
A multipactoring discharge is a discharge occurring in a vacuum across the RF electric field due to secondary electron emission on the electrode surface.
The conventional synchronous scan streaking device configuration and multipactoring discharge outline will be described hereafter.
FIG. 1 shows the cutaway view of the synchronous scan streaking device in the conventional device along the optical axis of the streaking tube structure.
Photoelectric layer 2 is formed on an inside surface at the bottom of tubular vacuum envelope 1, and phosphor layer 7 on the other inner surface.
A negative DC voltage with respect to a common reference potential is applied to phosphor layer 2 from power source E2.
Mesh electrode 3 is arranged adjacent to the photoelectric layer 2. A positive DC voltage with respect to photoelectric layer 2 is applied to mesh electrode 3 from power source E1 so as to accelerate photoelectrons generated from photoelectric layer 2.
Focusing electrode 4 is arranged in a space between anode plate 5 with an aperture at the center and the mesh electrode 3.
The anode plate 5 is connected to the common reference potential and a DC voltage supplied from power source E2 through a voltage divider appears at the focusing electrode 4. When the DC voltage is applied to the focusing electrode 4, an electron beam lens is formed to focus, on phosphor layer 7, photoelectrons generated from photoelectric layer 2.
A deflection voltage which periodically changes with time is applied across a pair of deflection electrode plates from deflection voltage generation means 8.
FIGS. 2A, 2B, 2C and 2D show scanning voltage waveforms together with images on phosphor layer 7, so as to illustrate the operation of the synchronous scan streaking device configuration in the conventional device.
The deflection voltage generation means 8 in the normal synchronous scan streaking device generates such a sine-wave voltage as shown in FIG. 2B, wherein linear portions p1 to q1, p2 to q2, ... pn to qn ... in the sine-waveform can be used to deflect the electron beam.
The sine-wave signal frequency is to be set at the same value as the repetition rate of the measured light pulses and the sine-wave signal phase is to be synchronizing with the measured light pulses.
Such a sine-wave signal voltage as shown in FIG. 2B is applied to deflection electrode plates 6a so as to observe such fluorescence as shown in FIG. 2A.
This sine-wave signal voltage can easily be obtained by generating another sine-wave signal voltage with the same phase at the same frequency, i.e., by using a laser beam generator to cause the fluorescence to occur.
FIG. 2C shows the light intensity distribution obtained along the time coordinate on phosphor layer 7 each time the electron beam is scanned.
The incident light beam intensity is diminished and thus the brightness changed along the time coordinate on phosphor layer 7 is very low when a deflection voltage changes along line p1 to q1. Image (1) in FIG. 2C shows this operation. One could hardly recognize this type of brightness using only the naked eye. Repetitive scanning operations increase the brightness distribution as shown in (2) and (3) of FIG. 2C. The enhanced brightness resulting from n-times of repetitive scanning operations is expected to approach n-times the brightness obtained by a single scanning operation, as shown in (n) of FIG. 2C. The background level with no signal being input in a certain condition, however, increases with the number of times of scanning operations, as shown in FIG. 2D. This increase might be caused by multipactoring discharge.
If the incident light pulses are clocked at a frequency of the order of hundred MHz in the VHF/UHF band, the sine-wave voltage to be used for scanning the electron beam should be of the order of hundred MHz.
When a VHF/UHF frequency RF voltage is applied across a pair of deflection electrodes, a multipactoring discharge can occur in a space adjacent to the deflection electrode and glass tube wall where a high frequency electric field is formed by the applied RF voltage. The multipactoring discharge area defined by S is enclosed within a broken line, as shown in FIG. 1.
The multipactoring discharge area is mainly defined by the deflection electrode plate 6a, the wall of the envelope 1, a deflection electrode plate lead connecting the deflection electrode plate 6a to the deflection voltage generation means 8 through the envelope 1, and the anode electrode 5, but it is not always limited to the area within these elements.
Light from scintillation occurring in space S, which is reflected from various portions within envelope 1, arrives at photoelectric layer 2 passing through an aperture across anode 5 and it may cause photoelectric layer 2 to generate parasitic photoelectrons.
Photoelectron emission due to any other than the signal component increases the background level on phosphor layer 7.
The multipactoring discharge excites electrons near the deflection electrode 6a within the space S. The excited electrons strike the deflection electrode plate at which an RF voltage is applied, the deflection electrode plate lead at which the deflection electrode plate is connected, the glass wall portion of envelope tube 1, and anode electrode 5. When an RF electromagnetic field is applied to the deflection electrode plate, the excited electrons may travel forward and back along complicated paths. Secondary electrons are emitted each time the excited electrons strike the above tube parts. As the secondary electrons increase, an avalanche breakdown may occur in space S causing a multipactoring discharge.
Edward F. Vance describes in a paper "One-Sided Multipactor Discharge Mode", Journal of Applied Physics, Vol. 34, No. 11, pp. 3237-3242 that the multiplactor discharge mode can be suppressed by limiting both the RF deflection signal frequency and amplitude to decrease secondary electrons from the surfaces of electrodes due to discharges among different electrodes.
Deflection electrode plate 6a, to which the deflection signal voltage is applied, in the streaking tube structure of the synchronous scan streaking device is arranged to form a discharge spaced (S in FIG. 1), together with a lead through which an external RF voltage is applied to the deflection electrode plate. In addition, these parts constitute a complicated structure to cause a multipactoring discharge, together with the glass wall and anode electrode 2 surrounding these parts.
The device structure, however, cannot easily be modified because of the complicated structure described above.
The signal voltage applied to the deflection electrode plates of the synchronous scan streaking device should have the same frequency as the light pulses to be observed, and it should change in the LF to VHF/UHF frequency range.
The scanning rate is directly proportional to the sine-wave frequency; and its scanning speed relates to the gradient of the voltage waveform and to the amplitude of the signal voltage. The signal amplitude should be set at a specific value to keep the scanning voltage linearity satisfactory with satisfactory time resolution.
Because of the above reason, the frequency and amplitude of the signal voltage applied to the deflection electrode plates cannot be limited in spite of Vance's theory cited heretofore.
In addition, alkaline metal vapor introduced into the tube during photoelectric layer fabrication adheres to the inner surface of the glass wall as well as the other electrode surfaces. This alkaline metal increases the secondary electron emissivity and makes a multipactoring discharge occur easily.
The objective of the present invention is to provide a new type of synchronous scan streaking device wherein the background level setup due to the multipactoring discharge is reduced drastically.